Optic design and manufacturing issues of a high numerical broadband IR solid immersion microscope objective

Abstract

Over the last two decades solid immersion objectives have been developed for various applications, offering the opportunity to achieve a higher resolution than is possible for conventional air objective lenses. For semiconductor applications hemispherical solid immersion lenses (SILs), incorporating tips fabricated in semiconductor materials with a high refractive index (up to n = 3:5 for silicon, for example) are commonly used. With such high refractive index materials it is possible to design an objective with a numerical aperture of NA = 3:2. An apochromatic color correction is mandatory if a broad spectral range from λ = 1200 nm to λ = 2000 nm is required. It is well known that glasses with anomalous partial dispersion must be used to realize apochromatic color correction. It will be shown that the anomalous partial dispersions of some glasses in the IR range differ from the known behavior in the visible region. Therefore, glass selection plays a significant role for the design of a high NA broadband IR objective and will be discussed. Monte Carlo tolerance analysis shows that even with state of the art manufacturing capabilities the tolerance induced aberrations of an objective with NA = 3:2 will lead to a dramatic loss of image performance, indicated by a significant drop in the Strehl-ratio. This becomes even worse when the interface of the exchangeable hemispherical SIL tip with the main objective is considered. With the aim to manufacture such objectives within a stable production process it makes no sense to overload the requirements regarding the NA. Therefore, due to manufacturing issues the numerical aperture of this objective has been restricted to NA = 2:9. Wave front measurements of the manufactured objectives shows Strehl-ratios of SR > 97 % which guarantees a diffraction limited resolution.